The present invention relates to a sheet feeding device capable of feeding a sheet such as paper, and an image forming apparatus using the sheet feeding device.
A general sheet feeding device used in an image forming apparatus or the like includes a pickup roller which comes in contact with a paper sheet, which is positioned on an upper end, in a stack direction, of a stack of a plurality of paper sheets held in a holding unit, such as a paper supplying cassette. The pickup roller rotates to transport the paper sheet to an image forming unit.
The pickup roller is configured to apply a transport force to a paper sheet positioned on the upper end in the stack direction while rotating in contact with the paper sheet. For this reason, as the position of the paper sheet positioned on the upper end in the stack direction is separated from the pickup roller due to the decrease in the number of the paper sheets in the holding unit, the contact surface pressure between the pickup roller and the paper sheet decreases. As a result, sufficient transport force is not applied to the paper sheet causing transport performance to be deteriorated.
Consequently, in order to reliably transport a sheet such as paper, it is preferable that the contact surface pressure between the pickup roller and the paper sheet positioned on the upper end in the stack direction is constantly maintained without large fluctuations.
In this regard, for example, a related apparatus maintains a substantially constant contact surface pressure by installing a paper pressing plate, which displaces all the paper upwardly so that all the loaded paper sheets remain close to the pickup roller as the number of the loaded paper sheets decreases.
Further, for example, another related apparatus maintains the contact surface pressure to be substantially constant by displacing a stationary end (an end fixed to a body side of an apparatus) of a tension spring which generates contact surface pressure in cooperation with an operation of a solenoid, wherein the solenoid displaces the pickup roller to bring the pickup roller in contact with the paper sheet or to separate the pickup roller from the paper sheet.